The present invention relates to an electronic camera apparatus and, more particularly, to an electronic camera apparatus for correcting the luminance level of a luminance signal obtained from an image sensing device having a color filter.
Generally, many electronic camera apparatuses called digital still cameras employ an arrangement for sensing a color image using a color filter formed on one image sensing device.
In this arrangement, an analog image signal is obtained from the image sensing device, in which a color image is represented by assigning predetermined colors to a number of pixels in advance, and pieces of luminance information with analog values representing the luminances of the pixels are discrete on the time axis.
Normally, in sensing a color image, the luminance of an obtained image signal must be corrected in accordance with the type of light source and ambient brightness.
In the electronic camera apparatus, a luminance correction amount is calculated from an image signal obtained by the image sensing device in, e.g., a test photometry mode before image sensing, and an image signal obtained later by pressing the shutter button is corrected using the luminance correction amount.
Assume a case wherein an image signal is converted into digital image data by an A/D converter and then subjected to various image processing operations. After the image signal is A/D-converted, information of details is lost by quantization. For this reason, the image quality obtained by subsequent image processing is largely influenced by the quality of original information.
Especially, a pixel with low luminance has a very small amount of information and is also greatly influenced by noise and the like. Normally, before A/D conversion, an analog signal is directly amplified to minimize damage to the information amount of such pixel with low luminance.
FIG. 16 shows a conventional electronic camera apparatus.
A color image sensed by an image sensing device 101 in the test photometry mode is input to a variable gain amplification section 102 as an image signal 111.
The image signal 111 is amplified by the gain controlled by a gain control section 103, converted into a digital signal by an A/D conversion section 104, and temporarily stored in a digital memory 105 as image data.
Image data 113 is read out from the digital memory 105. A luminance detection section 106 detects statistical luminance information such as average luminance and maximum and minimum luminance values in units of colors of the image data 113.
In accordance with the detection result from the luminance detection section 106, a control section 107 calculates a correction coefficient common to all colors. The gain of the variable gain amplification section 102 is set by the gain control section 103 on the basis of the correction coefficient.
When an image signal is obtained indoors under an incandescent lamp, the white balance of the obtained color image is bad, and the luminance of blue (B) tends to extremely lower, as shown in FIGS. 17A to 17D.
For example, it should be balanced as shown in FIG. 17A under ideal light source, the output signal from the image sensing device 101 are not balanced due to the influence of the incandescent lamp, as shown in FIG. 17B.
In this case, the control section 107 calculates a correction coefficient that corrects with which the luminance of blue (B) has an appropriate value to some degree, and a corresponding gain is set by the gain control section 103.
In the subsequent image sensing mode, pieces of luminance information contained in the image signal 111 from the image sensing device 101 are amplified by the gain based on the correction coefficient at the same magnification for all colors, as shown in FIG. 17C.
Then, as described above, the image signal is converted into a digital signal by the A/D conversion section 104 and temporarily stored in the digital memory 105 as new image data.
After that, new image data 113 is read out from the digital memory 105, and the statistical luminance information of the new image data 113 is detected by the luminance detection section 106 in the same way as described above.
Accordingly, the control section 107 determines whether image processing is necessary and calculates parameters therefor. An image processing section 108 executes predetermined image processing for the new image data 113, e.g., processing of further amplifying the luminance of blue (B).
With this processing, image data color-balanced to some extent is generated, as shown in FIG. 17D, and stored in the digital memory 105.
The image processing section 108 also performs, other processing such as color correction, contour extraction, and compression.
However, such a conventional electronic camera apparatus has the following problems because the variable gain amplifier provided on the output side of the image sensing device amplifies the image signal obtained from the image sensing device by the same gain for all colors and all pixel positions.
Referring to FIG. 16, if the image signal 111 obtained from the image sensing device 101 has extremely different luminances of red, green, and blue (RGB), a color that originally obtains satisfactory luminance is saturated when amplified by the variable gain amplification section 102. The luminance distribution of the image shifts to the bright side, resulting in poor image quality.
For example, referring to FIG. 17C, when the luminance of blue (B) is amplified to an appropriate value, red and green (RG) exceed the input range (0 to 1.0) of the A/D conversion section 104.
Hence, the luminance distribution shifts to the high luminance side. In addition, all the luminance values exceeding the input range become the highest luminance value (1.0), and the original luminance information is lost, resulting in an image with poor quality.
Normally, in the optical system used to focus light to the image sensing device, light at corners is attenuated as compared to the central portion due to the Cosine Fourth Law or shading, as known as vignetting.
However, the variable gain amplification section 102 of the conventional electronic camera apparatus shown in FIG. 16 amplifies the entire image region by the same gain and therefore cannot correct vignetting.
Such vignetting may be corrected by filter processing by the image processing section on the output side. In fact, vignetting tends not to be corrected because the processing takes a time and consumes power.